# Spotting a Black Hole in the Microwave Background

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• tionis
In summary: Yes, it is possible. Although it would be quite faint, it would be observable with sensitive instruments.Yes, it is possible. Although it would be quite faint, it would be observable with sensitive instruments.
tionis
Gold Member
If we were to take a microwave picture of a region of space said to have a black hole, would we be able to spot the black hole? Would we see a cold spot surrounded by a more hotter background?

A black hole in otherwise near-empty space so we won't see extraneous (and very obvious) phenomena like accretion disks of incandescent gas?

That ideal black hole is a black hole no matter what wavelength we're looking at. Even if you're considering that black hole will be radiating Hawking radiation, the effective temperature of the event horizon of a stellar-mass black hole is only a tiny fraction of a degree above absolute zero, and far colder than the cosmic background radiation around it.

tionis
If the cold spot were big enough to see, you're probably too close.

tionis
Nugatory said:
A black hole in otherwise near-empty space so we won't see extraneous (and very obvious) phenomena like accretion disks of incandescent gas?.

Yes, that is the scenario. So, a sat is mapping an empty region of space in microwaves, and it sees no significant temperature fluctuations except where the black hole is. Would the satellite capture a drop in the temperature where the BH is even if there are microwaves in the space between the sat and the BH? Would the picture reveal the shape of the BH?

If the cold spot were big enough to see, you're probably too close.
Even for a supermassive black hole that is not accreting anything?

If a SMBH had the angular size of the moon, you'd be about a billion miles away. The gravity would be about 20g's, and in a few hours your day is going to get very bad.

tionis
If a SMBH had the angular size of the moon, you'd be about a billion miles away. The gravity would be about 20g's, and in a few hours your day is going to get very bad.
Wow! I thought supermassive black holes tidal gravity was less destructive because of the size of the BH.

tionis said:
I thought supermassive black holes tidal gravity was less destructive

He's not talking about tidal gravity. He's talking about the proper acceleration you would need to maintain in order to "hover" at that altitude above the supermassive BH. You have to hover because at that distance there are no possible free-fall orbits around the hole.

tionis
PeterDonis said:
He's not talking about tidal gravity. He's talking about the proper acceleration you would need to maintain in order to "hover" at that altitude above the supermassive BH. You have to hover because at that distance there are no possible free-fall orbits around the hole.
Ah, Ok, I got it now. So, Peter, If the sat were to hover at that distance where the BH would look like the moon like Vanadium said, would it be able to capture a noticeable cool, round spot in the microwave background?

tionis said:
If the sat were to hover at that distance where the BH would look like the moon like Vanadium said, would it be able to capture a noticeable cool, round spot in the microwave background?

If it were looking directly towards the hole, yes, I believe so.

tionis
PeterDonis said:
If it were looking directly towards the hole, yes, I believe so.
It just occurred to me that micowaves near the black hole would shift to the blue end of the spectrum, no? Wouldn't that make the BH hotter than the background?

tionis said:
micowaves near the black hole would shift to the blue end of the spectrum, no?

As seen by an observer very far away, they would blueshift as they got close to the hole, but they would redshift again as they came back out. The net effect would be zero.

As seen by an observer close to the hole, such as you proposed (close enough for the hole to have the same angular size as the moon), there would be some net blueshift of the CMB radiation as it came to you around the sides of the hole. But it wouldn't be that much; and there would still be a huge cold spot right in front of you, since no microwaves would be coming from the hole itself.

tionis
Is it possible to see an Einstein ring in the CMB?

## 1. How can black holes be detected in the microwave background?

Black holes can be detected in the microwave background through a process called gravitational lensing. This occurs when the gravitational pull of a black hole bends and distorts the light passing by it, causing a characteristic pattern in the microwave background.

## 2. What is the significance of spotting black holes in the microwave background?

Spotting black holes in the microwave background can provide valuable information about the early universe and the formation of galaxies. It can also help us understand the distribution of matter and energy in the universe.

## 3. How do scientists differentiate between black holes and other objects in the microwave background?

Scientists use specialized instruments, such as telescopes and satellites, to analyze the polarization and temperature fluctuations in the microwave background. This allows them to distinguish between the unique signature of a black hole and other objects in the background.

## 4. Can black holes be detected in the microwave background from any location in the universe?

Yes, black holes can be detected in the microwave background from any location in the universe. However, the level of detail and accuracy may vary depending on the capabilities of the instruments being used.

## 5. How does spotting black holes in the microwave background contribute to our understanding of the universe?

Studying black holes in the microwave background can help us understand the evolution of the universe, the nature of dark matter and dark energy, and the role of gravity in shaping the structure of the universe. It can also provide insights into the origins of the universe and its ultimate fate.

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